This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:    ACK/NACK acknowledge/not (negative) acknowledge    CDMA code division multiple access    CQI channel quality indicator    SDR software defined radio    SIR signal-to-inference ratio    QoS quality of service
In IEEE Journal on Selected Areas in Communications, VOL. 23, NO. 2, pp. 201-220 February 2005, Cognitive Radio: Brain-Empowered Wireless Communications, Simon Haykin offered the following definition of cognitive radio:
“Cognitive radio is an intelligent wireless communication system that is aware of its surrounding environment (i.e., outside world), and uses the methodology of understanding-by-building to learn from the environment and adapt its internal states to statistical variations in the incoming RF stimuli by making corresponding changes in certain operating parameters (e.g., transmit-power, carrier-frequency, and modulation strategy) in real-time, with two primary objectives in mind: highly reliable communications whenever and wherever needed; efficient utilization of the radio spectrum.”
Without effective avoidance of primary users, cognitive radio networks are unlikely to succeed. In a cognitive radio network there are primary users that communicate on a given frequency channel (for example), and it is desirable that the communication between the primary users not be disturbed to any significant extent. However, for efficient spectrum use the communication channels should be reused whenever possible by a pool of secondary users. When the secondary users communicate they should attempt to avoid the primary users as much as possible.